The present embodiments relate to a magnetic resonance tomography (MRT) system, in which a current for a balancing or shim coil may be generated as a function of an item of information relating to a field inhomogeneity in a magnetic field of the MRT system.
Imaging in magnetic resonance tomography (e.g., magnetic resonance imaging (MRI)) is based on an alignment of spins of atomic nuclei through a magnetic base field or B0 field of the MRT system. Attempts are made to provide as homogenous a base field as possible that has a constant field strength (e.g., both with respect to amount and direction) in a relatively large area.
The homogeneity of the base field is important for many applications for the image quality and also for the spatial registration of the images (e.g., the assignment of image regions that correspond to one another in two or more images). In order, for example, to improve the diagnostic useability of a magnetic resonance image (MR image), the fat saturation enables the relatively strong signals from fat tissue to be filtered out. With the spectral fat saturation and the techniques used, the fact that fat and water have slightly different resonant frequencies is utilized. The deviation is however minimal. The deviation may amount to 3.1 ppm (parts per million), which provides that with a Lamor or resonant frequency of 10 MHz, there is only a difference of 31 Hz. With this weak deviation, the signal of the fat may be suppressed through a strong transmit pulse at the fat frequency without influencing the imaging of the protons belonging to the water molecules. The functionality of all techniques that are based on the spectral separation of fat and water also depends on the homogeneity of the base field. If the base field varies in a similar order of magnitude to the spectral separation of fat and water (e.g., approximately 3 ppm), then the fat and water resonances may be at the same frequency. As a result, the fat and water resonances may no longer be spectrally separated.
With current superconducting magnets, a structurally-specific magnetic field inhomogeneity may be reduced to approximately 1 ppm and less in a volume of approximately 30×40×50 cm. Problems with fat saturation results, for example, in border areas of these regions if a patient to be examined has wide shoulders, for example, that protrude from the volume with a homogenous field. In order to enlarge the volume with the homogenous magnetic field, in addition to the base field generated by the superconducting magnetic coil, this base field may be overlaid with a balancing field using balancing or shim coils. The inhomogeneities may be compensated for at the borders of the volume using the balancing or shim coils. Such shim coils may be fastened to the superconducting magnets. The shim coils are supplied with an electrical current by a shim current source in each instance, the current strength of which is set within the scope of adjusting the MRT system by the service personnel.
The inhomogeneities introduced by the tissue of a patient are more critical than the deterministic B0 inhomogeneities that may be measured in advance. For example, discontinuities of air and tissue result in significant B0 distortions. The inhomogenous distribution of water, air, bones and fat in the human body also result, on account of the spatial susceptibility distribution, in a distortion of the B0 base field that differs for each patient.